Differential geared amplification of auxiliary power unit

ABSTRACT

A disclosed drive assembly for an auxiliary power unit includes a first drive shaft driven by an auxiliary power unit, a second drive shaft driven by a first electric motor, a differential gear system including a ring gear driven by the second drive shaft, a and planet gears supported within a carrier attached to the ring gear. The first drive shaft and an output shaft are coupled to the planet gears and a generator is driven by the output shaft. The electric motor and the auxiliary power unit combine to drive the output shaft.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 16/145,431filed on Sep. 28, 2018.

BACKGROUND

A gas turbine engine typically includes a fan section, a compressorsection, a combustor section and a turbine section. Air entering thecompressor section is compressed and delivered into the combustionsection where it is mixed with fuel and ignited to generate a high-speedexhaust gas flow. The high-speed exhaust gas flow expands through theturbine section to drive the compressor and the fan section. Thecompressor section typically includes low and high pressure compressors,and the turbine section includes low and high pressure turbines.

Auxiliary power units are small gas turbine engines that are used todrive accessories such as electric generators or other accessory drivesystems. Operation of the gas turbine engine is most efficient at fixedoperating conditions. Differing loads on the auxiliary power units canreduce efficiency by moving operation away from the most efficientoperating conditions.

Turbine engine manufacturers continue to seek further improvements toengine performance including improvements to thermal, transfer andpropulsive efficiencies.

SUMMARY

A drive assembly for an auxiliary power unit according to an exemplaryembodiment of this disclosure includes, among other possible things, afirst drive shaft driven by an auxiliary power unit; a second driveshaft driven by a first electric motor; a differential gear systemincluding a ring gear driven by the second drive shaft, planet gearssupported within a carrier attached to the ring gear wherein the firstdrive shaft and an output shaft is coupled to the planet gears; and agenerator driven by the output shaft, wherein the electric motor and theauxiliary power unit combine to drive the output shaft.

In a further embodiment of the foregoing drive assembly for an auxiliarypower unit, a speed of the first electric motor is varied to adjust aspeed of the output shaft and the speed of the first drive shaft isconstant over a predefined operating range.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the generator provides electric power to a secondelectric motor coupled to a spool of a gas turbine engine.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the auxiliary power unit includes a firstcompressor coupled to a first turbine and the first drive shaft.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the first drive shaft and first compressor aredisposed along an APU rotational axis.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the first drive shaft and the first turbine aredisposed along an APU rotational axis.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the second drive shaft is disposed along an axistransverse to the APU rotational axis.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the output shaft is coupled to drive an auxiliarycompressor.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the auxiliary compressor is in communication witha turbine section of a gas turbine engine.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, the auxiliary compressor is in communication withan environmental control system of an aircraft.

In another embodiment of any of the foregoing drive assemblies for anauxiliary power unit, a controller that controls operation of theauxiliary power unit and the first electric motor provides a predefinedspeed of the output shaft.

A gas turbine engine assembly according to an exemplary embodiment ofthis disclosure includes, among other possible things, a main gasturbine engine including a compressor section communicating compressedair to a combustor section where fuel is combined with the compressedair and ignited to generate a high energy flow that expands through aturbine section, wherein the turbine section is coupled to thecompressor section by a main shaft; an auxiliary power unit including anAPU compressor coupled to an APU turbine section along an APU driveshaft; a differential gear system including a ring gear and planet gearssupported within a carrier attached to the ring gear, wherein the APUdrive shaft is engaged to the planet gears; a first drive means engagedto drive the ring gear; an output shaft engaged to the planet gears; agenerator driven by the output shaft; and a second drive means coupledto the main shaft of the main gas turbine engine for applying a partialdrive load to the main shaft.

In a further embodiment of the foregoing gas turbine engine assembly,the output shaft drives an auxiliary compressor, and the auxiliarycompressor provides supplemental flow to the turbine section.

In another embodiment of any of the foregoing gas turbine engineassemblies, the first drive means comprises a first electric motorcoupled through a first drive shaft to drive the ring gear at apredefined speed to provide a predefined output speed when combined withan output speed provided by the first drive shaft driven by theauxiliary power unit.

In another embodiment of any of the foregoing gas turbine engineassemblies, the second drive means comprises a second electric motorcoupled to the main shaft through a gear coupling.

In another embodiment of any of the foregoing gas turbine engineassemblies, a controller controls a rotational speed of the output shaftby varying an input speed of the first drive shaft driving the ring gearand the APU drive shaft driving the planet gears.

In another embodiment of any of the foregoing gas turbine engineassemblies, operation of the first drive means and the second drivemeans varies for selectively varying loads on the auxiliary power unitand the main gas turbine engine.

A method of operating a gas turbine engine according to an exemplaryembodiment of this disclosure includes, among other possible things,driving a planet gear of a differential gearbox with an APU shaft drivenby an auxiliary power unit; driving a ring gear of the differentialgearbox with a first electric motor; and varying a speed of an outputshaft from the differential gearbox by adjusting a speed of the electricmotor while maintaining a constant speed of the APU shaft.

In a further embodiment of the foregoing method of operating a gasturbine engine, the output shaft drives the load compressor andselectively supplies bleed airflows from the load compressor to a mainengine. The engine comprises a gas turbine engine including a maincompressor section and a main turbine section coupled through a mainshaft, and the bleed airflows from the load compressor are communicatedto the main turbine.

In another embodiment of any of the foregoing gas turbine engineassemblies, the output shaft drives an electric generator and a secondelectric motor coupled to the main shaft of the main engine.

In another embodiment of any of the foregoing gas turbine engineassemblies, the output shaft drives a load compressor and selectivelysupplies bleed airflows from the load compressor to an aircraftenvironmental control system.

In another embodiment of any of the foregoing gas turbine engineassemblies, a speed of the APU shaft and a speed of the first electricmotor varies to control a speed of the output shaft.

Although the different examples have the specific components shown inthe illustrations, embodiments of this invention are not limited tothose particular combinations. It is possible to use some of thecomponents or features from one of the examples in combination withfeatures or components from another one of the examples.

These and other features disclosed herein can be best understood fromthe following specification and drawings, the following of which is abrief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example gas turbine engine.

FIG. 2 is a schematic view of an example auxiliary power unit assemblyembodiment and a portion of the example gas turbine engine.

FIG. 3 is a graph illustrating relationships between rotational speedsof inputs and outputs of the example auxiliary power unit assembly.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. The fan section 22 drivesair along a bypass flow path B in a bypass duct defined within a nacelle15, and also drives air along a core flow path C for compression andcommunication into the combustor section 26 then expansion through theturbine section 28. Although depicted as a two-spool turbofan gasturbine engine in the disclosed non-limiting embodiment, it should beunderstood that the concepts described herein are not limited to usewith two-spool turbofans as the teachings may be applied to other typesof turbine engines including three-spool architectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects, a first (or low) pressure compressor 44 and a first (orlow) pressure turbine 46. The inner shaft 40 is connected to a fan 42through a speed change mechanism, which in exemplary gas turbine engine20 is illustrated as a geared architecture 48 to drive the fan 42 at alower speed than the low speed spool 30. The high speed spool 32includes an outer shaft 50 that interconnects a second (or high)pressure compressor 52 and a second (or high) pressure turbine 54. Acombustor 56 is arranged in exemplary gas turbine 20 between the highpressure compressor 52 and the high pressure turbine 54. A mid-turbineframe 58 of the engine static structure 36 may be arranged generallybetween the high pressure turbine 54 and the low pressure turbine 46.The mid-turbine frame 58 further supports bearing systems 38 in theturbine section 28. The inner shaft 40 and the outer shaft 50 areconcentric and rotate via bearing systems 38 about the engine centrallongitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 58 includes airfoils 60 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and fan drive gear system 48 may be varied. For example,gear system 48 may be located aft of the low pressure compressor, or aftof the combustor section 26 or even aft of turbine section 28, and fan42 may be positioned forward or aft of the location of gear system 48.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1 and less than about 5:1. Itshould be understood, however, that the above parameters are onlyexemplary of one embodiment of a geared architecture engine and that thepresent invention is applicable to other gas turbine engines includingdirect drive turbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet (10,668 meters). The flight condition of 0.8 Mach and35,000 ft (10,668 meters), with the engine at its best fuelconsumption—also known as “bucket cruise Thrust Specific FuelConsumption (‘TSFC’)”—is the industry standard parameter of lbm of fuelbeing burned divided by lbf of thrust the engine produces at thatminimum point. “Low fan pressure ratio” is the pressure ratio across thefan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The lowfan pressure ratio as disclosed herein according to one non-limitingembodiment is less than about 1.45. “Low corrected fan tip speed” is theactual fan tip speed in ft/sec divided by an industry standardtemperature correction of [(Tram ° R)/(518.7° R)]0.5. The “Low correctedfan tip speed” as disclosed herein according to one non-limitingembodiment is less than about 1150 ft/second (350.5 meters/second).

The example gas turbine engine includes the fan 42 that comprises in onenon-limiting embodiment less than about 26 fan blades. In anothernon-limiting embodiment, the fan section 22 includes less than about 20fan blades. Moreover, in one disclosed embodiment the low pressureturbine 46 includes no more than about 6 turbine rotors schematicallyindicated at 34. In another non-limiting example embodiment, the lowpressure turbine 46 includes about 3 turbine rotors. A ratio between thenumber of fan blades 42 and the number of low pressure turbine rotors isbetween about 3.3 and about 8.6. The example low pressure turbine 46provides the driving power to rotate the fan section 22 and thereforethe relationship between the number of turbine rotors 34 in the lowpressure turbine 46 and the number of blades 42 in the fan section 22disclose an example gas turbine engine 20 with increased power transferefficiency.

The example gas turbine engine 20 includes an auxiliary power unitassembly 16 that drives a generator 66. The generator 66 in turnprovides electric power to an aircraft electric power network 110 and anelectric motor 64 coupled to the high speed spool 32. The auxiliarypower unit assembly 16 includes an auxiliary power unit 62 that drives adrive system 70 that in turn drive the generator 66. The drive system 70may also drive an auxiliary gearbox 72 utilized to drive accessorydevices such as hydraulic pumps, fuel pumps, lubricant pumps and otheraccessory devices utilized to support operation of the gas turbineengine 20. The drive system 70 may also drive an auxiliary compressor74. The drive system 70 is driven by inputs from both the auxiliarypower unit 62 and a first drive means 68. The first drive means 68 is anelectric motor that aids in adjusting and varying a speed of an outpututilized to drive the generator 66 and the auxiliary power unit 62.

The electric motor 64 coupled to the high speed spool 32 can be operatedto start the engine 20. The electric motor 64 may also be utilized toaccommodate variations in loads on the high speed spool 32 during engineoperation.

Referring to FIG. 2 with continued reference to FIG. 1 , the exampleauxiliary power unit assembly 16 is coupled to the drive system 70 thatis a differential gearbox. The drive system 70 includes a ring gear 96that is driven by a pinion gear 98 driven by the electric motor 86. Theelectric motor 86 drives a first shaft 90 that is coupled to the ringgear 96 by the pinion gear 98. A clutch 88 is provided between the firstshaft 90 and the electric motor 86 such that electric motor 86 may notbe back driven by the drive system 70. Only rotation in one direction iscommunicated to drive the ring gear 96. In this example, the clutch 88is a mechanical sprag clutch that does not require controller input foractuation.

The disclosed example auxiliary power unit 62 includes an APU compressor82 that compresses air and communicates that air to an APU combustor 84.In the APU combustor 84 fuel is mixed with the compressed air andignited to generate a high energy exhaust gas flow. The high energyexhaust gas flow expands through an APU turbine 80 that drives thecompressor 82 through the shaft 92. Variations on loads on the auxiliarypower unit 62 are conventionally accommodated by adjusting fuel flow tochange operation. The example auxiliary power unit assembly 62 uses theelectric motor 86 to accommodate some of the variations in load so thatthe auxiliary power unit 62 can operate at a sustained and efficientspeeds. It should be appreciated that although a gas turbine auxiliarypower unit is disclosed by way of example, other engines and devicescould be utilized with the disclosed drive system 70 and are within thecontemplation and scope of this disclosure.

Optionally, a clutch 89 is provided between the shaft 92 and the drivesystem 70 such that APU 62 may not be back driven by the drive system70. Only rotation in one direction is communicated to drive the shaft92. In this example, the clutch 89 is a mechanical sprag clutch thatdoes not require controller input for actuation.

The ring gear 96 supports a carrier 102 that supports planet gears 100.The planet gears 100 are coupled to a pinion gear 104 coupled to an APUshaft 92 and a pinion gear 106 coupled to the output shaft 94. The APUshaft 92 and output shaft 94 are disposed along a common APUlongitudinal axis 78. The output shaft 94 drives the generator 66 and anauxiliary compressor 74. The auxiliary compressor 74 is also referred toas a load compressor 74. In this example the generator 66 and auxiliarycompressor, 74 are disposed along the APU axis 78. However, otherconfigurations and relative orientations of the auxiliary compressor 74and the electric generator 66 are within the scope and contemplation ofthis disclosure.

Rotation of the APU shaft 92 and the first shaft 90 driven by theelectric motor 86 combine to provide the output speed of the outputshaft 94 that drives the generator 66. Accordingly, the speed of theauxiliary power unit 62 may operate at a speed different than the speedof the output shaft 94. The different operating speeds enable operationof the auxiliary power unit 62 at a more efficient speed while enablingoperation of the electric generator 66 at higher efficient speeds. Theelectric motor 86 drives the first shaft 90 to enable variations in thespeed of the output shaft 94 to accommodate changes in loads rather thanchanging the operating speed of the auxiliary power unit 62 away fromefficient operating speeds.

The electric generator 66 powers a second electric motor 64 that iscoupled through a gear coupling schematically shown at 65 to the highspeed spool 32. The electric motor 64 provides an additional drive toaccommodate changes in load on the high speed spool 32 during operation.The added torque to the high speed spool 32 provided by the electricmotor 64 can reduce variations in loads on the high speed spool 32 thatcan reduce engine efficiencies. The electric motor 64 can increase ordecrease torque applied to the high speed spool 32 to accommodate engineoperation without changing operation of the high speed spool 32. Itshould be appreciated that although the high speed spool 32 is disclosedand explained by way of example, that the low speed spool 30 or otherintermediate spools of a gas turbine engine could also be coupled to anelectric motor either instead of the high speed spool 32 or in additionto the high speed spool 32.

Additionally, the output shaft 94 drives the auxiliary compressor 74 ata speed different than that of the auxiliary power unit 62. Inlet guidevanes shown schematically at 108 direct airflow into the auxiliarycompressor 74. The auxiliary compressor 74 in turn provides compressedair to an aircraft cabin environmental control system (ECS) 120 viableed air passage 122. The auxiliary compressor 74 is in communicationwith bleed air passage 122 that supplies air to the ECS. Valve 124governs the flow of bleed airflow to the ECS. The auxiliary compressor74 is in communication with bleed air passages 112 and 114 that supplybleed airflow to the high pressure turbine 54. Valves 116 and 118 governthe flow of bleed airflow to the high pressure turbine 54 to increaseoperational efficiency.

The bleed airflows 112, 114 supplement the high pressure turbine 54cooling air flow and increase the efficiency of turbine cooling athigher turbine inlet temperatures such as at aircraft takeoff and climboperations. The bleed airflow through air passage 112 enters highpressure turbine 54 downstream of the choked flow condition in the highpressure turbine 54 and does not substantially affect the pressure ratioand flow of high pressure compressor 52.

The bleed airflow through air passage 114 enters high pressure turbine54 upstream of the choked flow condition in the high pressure turbine 54and does substantially affect the pressure ratio and flowcharacteristics of high pressure compressor 52. Increasing bleed airflowthrough air passage 114 reduces airflow of high pressure compressor 52and increases a pressure ratio of high compressor 52 and increasesengine 20 efficiency at lower power such as for example at the bucketcruise condition. Decreasing bleed airflow through air passage 114increases airflow of high pressure compressor 52 and reduces thepressure ratio of high compressor 52 and increases engine 20 power asneeded at the aircraft takeoff and climb conditions.

The reduction in pressure ratio of the high compressor 52 reduces theinlet temperature to the combustor 26 and enables a larger temperaturerise across the combustor 26 to increase engine 20 power output asneeded at the aircraft takeoff and climb conditions. Inlet guide vanes108 control the airflow of auxiliary compressor 74 in response to bleedairflows 112, 114, and 122 and the speed of shaft 94 to maintain higherefficiency of the auxiliary compressor 74.

The APU 62 and electric motor 86 drive auxiliary compressor 74. Theauxiliary compressor 74 supplements the flow and varies the flow andpressure ratio of high compressor 52. The APU 62 and electric motor 64,through the impact of bleed airflow from auxiliary compressor 74,enhance the operation of engine 20, and the APU 62 and electricgenerator 66, through the impact of torque from electric motor 64,enhance the operation of engine 20. The electric motor 86 and drivesystem 70 enhance the operation of APU 62 as APU assembly 16 enhancesthe operation of engine 20. The controller 76 adjusts operation of theelectric motor 86 and the auxiliary power unit 62 to provide the desiredpower and bleed airflow output of APU assembly 16 and maintains theauxiliary power unit 62 at efficient and steady state conditions forlonger periods of operation.

Referring to FIG. 3 with continued reference to FIG. 2 , graph 130illustrates a relationship between the inputs provided by the electricmotor 86 and the auxiliary power unit 62 to the drive system 70 and theoutput provided to the output shaft 94. In this example, the directionof rotation of the auxiliary power unit 62 is opposite the rotation ofthe output shaft 94. This is indicated by the different directions ofthe arrows shown on the axes of the graph 130. The different lines136A-D represent rotational speeds of the electric motor 86. As isshown, the relationship between the input speed indicated at 132 fromthe auxiliary power unit 62 and the speed 134 of the output shaft 94 issubstantially linear. The specific relationship is dependent on thespecific gear ratio provided by the drive system 70. It should beunderstood that any gear ratio between the APU shaft 92 and the outputshaft 94 that provides the desired operational speed of the generator 66and the auxiliary power unit 62 could be utilized and is within thecontemplation of this disclosure.

The first line 136A is indicative of relationship of speeds with theelectric motor 86 fixed such that no speed or torque is added by theelectric motor 86. Moving up to, the second line 136B illustrates howthe addition of power from the electric motor 86 will increase theoutput speed of the output shaft 94 without increasing the speedprovided by the APU shaft 92. A further increase in speed of theelectric motor 86 illustrated by the third line 136C shows the furtherincrease in output speed shown at 134 without additional power or speedprovided by the auxiliary power unit 62. A fourth line 136D illustratesa further increase in speed and power input from the electric motor 86to provide different and increasing output speeds to the output shaft94. As appreciated, the speed of the electric motor 86 may be variedoutside of the lines shown in the graph 130 to adjust the output speedto drive the generator without changing operation of the auxiliary powerunit 62.

Accordingly, in operation, the auxiliary power unit 62 will be operatedat a predefined speed to provide a desired output speed of the outputshaft 94 to drive the generator 66. As loads on the generator 66 change,the electric motor 86 can be operated to add power and increase ordecrease speeds without adjusting operation of the auxiliary power unit62. When the demand for power and speed increases beyond that capable ofbeing accommodated by the electric motor 86, the auxiliary power unit 62is adjusted to provide increased speed and power. The controller 76adjusts operation of the electric motor 86 and the auxiliary power unit62 to provide the desired power output through the output shaft 94 tomaintain the auxiliary power unit at efficient and steady stateconditions for longer periods of operation.

When the shaft 92 speed is zero and the APU 62 is not operating, theelectric motor 86 and drive system 70 can provide power to auxiliarycompressor 74 and electric generator 66 as needed by engine 20 and theECS 120. APU 62 may be selectively operated or not operated at all.Additionally, the electric motor 86 and drive system 70 can providepower to auxiliary compressor 74 and electric generator 66 as needed byengine 20 and the ECS 120 in the event of failure of APU 62.

Additionally, loads on the high speed spool 32 can be aided by theelectric motor 64 that draws power from the generator 66. Increasedloads on the high spool 32 can be first accommodated by increased powerinput from the electric motor 64. The variations in power required bythe electric motor 64 are accommodated by adjustments in the electricmotor 86 and auxiliary power unit 62 through the two inputs to the drivesystem 70.

Additionally, bleed from auxiliary compressor 74 to ECS 120 can be firstaccommodated by increased power input from the electric motor 86. Thevariations in bleed flow required by the ECS 120 are accommodated byadjustments in the electric motor 86 and auxiliary power unit 62 throughthe two inputs to the drive system 70.

Accordingly, the example auxiliary power unit assembly 16 providesadditional power and torque to accommodate variations in power and loadswhile enabling operation of the auxiliary power unit 62 and gas turbineengine 20 at more efficient settings.

Although an example embodiment has been disclosed, a worker of ordinaryskill in this art would recognize that certain modifications would comewithin the scope of this disclosure. For that reason, the followingclaims should be studied to determine the scope and content of thisdisclosure.

What is claimed is:
 1. A gas turbine engine assembly comprising: a maingas turbine engine including a compressor section communicatingcompressed air to a combustor section where fuel is combined with thecompressed air and ignited to generate a high energy flow that expandsthrough a turbine section, wherein the turbine section is coupled to thecompressor section by a main shaft; an auxiliary power unit including anAPU compressor coupled to an APU turbine section along an APU driveshaft; a differential gear system including a ring gear and planet gearssupported within a carrier attached to the ring gear, wherein the APUdrive shaft is engaged to the planet gears; a first drive means engagedto drive the ring gear; an output shaft engaged to the planet gears; agenerator driven by the output shaft; a second drive means coupled tothe main shaft of the main gas turbine engine for applying a partialdrive load to the main shaft; and a controller configured to controloperation of the first drive means, the auxiliary power unit and thegenerator, where the controller is configured to: based on adetermination that an increase in a demand for power on the generator isbelow a threshold demand, increase a speed of the first drive means toincrease a power output from the generator to meet the demand whilemaintaining the APU at a constant speed; and based on a determinationthat the demand for power is above the threshold, increase the speed ofthe APU to increase the power output from the generator to meet thedemand for power.
 2. The gas turbine engine as recited in claim 1,including an auxiliary compressor driven by the output shaft, theauxiliary compressor providing supplemental flow to the turbine section.3. The gas turbine engine as recited in claim 2, wherein the auxiliarycompressor is in communication with an environmental control system ofan aircraft.
 4. The gas turbine engine as recited in claim 1, whereinthe first drive means comprises a first electric motor coupled through afirst drive shaft to drive the ring gear at a predefined speed toprovide a predefined output speed when combined with an output speedprovided by the APU drive shaft driven by the auxiliary power unit. 5.The gas turbine engine as recited in claim 4, wherein the first driveshaft is disposed along an axis transverse to the APU rotational axis.6. The gas turbine engine as recited in claim 4, wherein the seconddrive means comprises a second electric motor coupled to the main shaftthrough a gear coupling.
 7. The gas turbine engine as recited in claim1, including varying operation of the first drive means and the seconddrive means for selectively varying loads on the auxiliary power unitand the main gas turbine engine.
 8. The gas turbine engine as recited inclaim 1, wherein the APU compressor and APU turbine are coupled to theAPU drive shaft.
 9. The gas turbine engine as recited in claim 7,wherein the APU drive shaft, the APU turbine and the APU compressor aredisposed along the APU rotational axis.
 10. The gas turbine engine asrecited in claim 1, further including a first clutch provided on the APUdrive shaft configured to selectively decouple the APU drive shaft fromthe differential gear system for preventing back driving of theauxiliary power unit.
 11. The gas turbine engine as recited in claim 10,further including a second clutch disposed on a first drive shaft,coupled to the first drive means, configured to selectively decouple thefirst drive shaft from the differential gear system for preventing backdriving of the first drive means.
 12. The gas turbine engine as recitedin claim 11, wherein the first clutch and the second clutch areconfigured to automatically decouple the corresponding one of the APUdrive shaft and the first drive shaft from the differential gear systemwithout input from the controller.
 13. The gas turbine engine as recitedin claim 1, further comprising a first drive shaft coupled to the firstdrive means wherein the controller is further configured to provide adesired bleed airflow from an auxiliary compressor by adjusting a speedof the first drive shaft.
 14. The gas turbine engine as recited in claim1, wherein the controller is further configured to control the firstdrive means to drive the differential gear system when the auxiliarypower unit is not rotating the APU drive shaft.
 15. A method ofoperating a gas turbine engine comprising: providing a main gas turbineengine including a compressor section communicating compressed air to acombustor section where fuel is combined with the compressed air andignited to generate a high energy flow that expands through a turbinesection, wherein the turbine section is coupled to the compressorsection by a main shaft; an auxiliary power unit including an APUcompressor coupled to an APU turbine section along an APU drive shaft; adifferential gearbox including a ring gear and planet gears supportedwithin a carrier attached to the ring gear, wherein the APU drive shaftis engaged to the planet gears; a first electric motor engaged to drivethe ring gear; an output shaft engaged to the planet gears; a generatordriven by the output shaft; a second drive means coupled to the mainshaft of the main gas turbine engine for applying a partial drive loadto the main shaft; and a controller configured to control operation ofthe first drive means, the auxiliary power unit and the generator, wherethe controller is configured to: based on a determination that anincrease in a demand for power on the generator is below a thresholddemand, increase a speed of the first drive means to increase a poweroutput from the generator to meet the demand while maintaining the APUat a constant speed; and based on a determination that the demand forpower is above the threshold, increase the speed of the APU to increasethe power output from the generator to meet the demand for power;driving the planet gears of the differential gearbox with the APU shaftdriven by the auxiliary power unit; driving the ring gear of thedifferential gearbox with the first electric motor; and varying thespeed of the output shaft from the differential gearbox by adjusting thespeed of the electric motor while maintaining the constant speed of theAPU shaft.
 16. The method as recited in claim 15, including driving aload compressor with the output shaft and selectively supplying bleedairflows from the load compressor to the turbine section.
 17. The methodas recited in claim 16, wherein the second drive means is a secondelectric motor including driving an electric generator with the outputshaft and driving the second electric motor coupled to the main shaft ofthe main gas turbine engine.
 18. The method as recited in claim 17,including selectively supplying bleed airflows from the load compressorto an aircraft environmental control system.
 19. The method as recitedin claim 18, including varying the speed of the APU shaft and the speedof the first electric motor to control a speed of the output shaft.